Device for separating two liquids of different densities in a tank containing these two liquids

ABSTRACT

A device for separating two liquids of different densities in a tank containing these two liquids one above the other which includes a separating element floating with clearance along the vertical wall of the tank between the two liquids, and a flexible sealing skirt of reduced thickness adapted to tangentially contact the vertical wall of the tank and to turn upwardly or downwardly along the tank wall without mixing of the two liquids when the tank is switched from a filling to an emptying operation or vice versa. The separating element may be provided with openings for passages of pipes, the edges of which are also provided with flexible sealing skirts, for filling or emptying the tank. Also, the separating element may include grooves for draining decanting products from the upper liquid toward the lower liquid.

United States Patent [151 3,680,729

Bonavent et al. 1 1 Aug. 1, 1972 [54] DEVICE FOR SEPARATING TWO2,307,508 1/1943, Jayne ..220/26 S LIQUIDS OF DIFFERENT DENSITIES2,614,717 10/1952 Wiggins ..220/26 D IN A TANK CONTAINING THESE TWO593,333 11/1897 Park ..220/93 UX LIQUIDS 2,924,350 2/1960 Greer ..220/l3Inventors: Gerard Bonavent Rue M almaison; 3,049,261 8/1962 Wade et a1..220/26 S Michel Huvey Bougiv Marcel 3,159,301 12/1964 Anderson..220/26 S 9 s Peinado, Maisoniafitm all of 3,164,289 l/ 1965Cocchlarella ..220/93 France Primary Examiner-Joseph R. Leclair [73]Assignee: Institut Francais Du Petrole Des Assi tan Examiner-James R.Garrett Carburants Et Lubrifiants, Rueil At yg Antonelli & HillMalmaison (l-lauts de Seine), France [57] ABSTRACT [22] Filed: Dec. 23,1969 A device for separating two liquids of different densities in atank containing these two liquids one above [21] Appl' 887669 the otherwhich includes a separating element floating with clearance along thevertical wall of the tank [30] Foreign Application Priority Data betweenthe two liquids, and a flexible sealing skirt of reduced thicknessadapted to tangentially contact the Dec. 24, 1968 France ..68l80573vertical wall of the k d t t m upwardly or downwardly along the tankwall without mixing of the U-S- Clo "220/229 when is from a to [51] IntCl 865d an emptying operation or vice versa. The separating element maybe provided with openings for passages [58] Field of zi gg g g of pipes,the edges of which are also provided with flexible sealing skirts, forfilling or emptying the tank. Also, the separating element may includegrooves for [56] References Cited draining decanting products from theupper liquid UNITED STATES PATENTS toward the lower q 3,438,215 4/1969Frijlink..... ..'....."....220/26 D 6 Claims, 9Drawing FiguresPATENTEDAUB 1 I972 SHEET 1 BF 3 FIGJ PATENTEmus 1 m2 3.680.729

sum 2 BF 3 FIGS DEVICE FOR SEPARATING TWO LIQUIDS OF DIFFERENT DENSITIESIN A TANK CONTAINING THESE TWO LIQUIDS It is known that a substantialcost reduction can be obtained in the building of large diarnetered andvery high tanks, when the stored product is immiscible or only slightlymiscible with water and higher than water, by giving these tankssubstantially the shape of a cup turned upside down and at least partlyimmersed in water The hydrostatic stresses exerted on the periphery ofthe tank are then reduced substantially and, consequently, the thicknessof the tank walls can be substantially reduced.

It is also known that savings are achievable in the field ofhydrocarbons delivery on shore by using a fluid under pressure, such as,for example, water, for permanently replenishing the volume which isleft free in the tanks by hydrocarbons and for providing thedisplacement of the hydrocarbons which are to be delivered.

One of the main difficulties encountered is however that microbialcolonies rapidly develop at the surface separating the two phases,specially when these two phases respectively consist of crude oil andsea water. Some of these colonies include bacteries whereby the sulfateswhich are normally present in water can be reduced and convertedtohydrogen sulfide (H 8), the latter being dissolved into the crude oiland being capable of developing an important corrosive action on thetank itself, when this tank is made of ordinary non-protected steel orof any other material which can be attacked by H 8, and of corroding allthe elements made of attackable material with which the crude oil maysubsequently come into contact, such as pipes, ship tanks, onshoretanks, heat exchangers and boiler piping, before the first distillationof the crude oil.

Furthermore this dissolution phenomenon increases the sulphur content ofthe crude oil, which has preferably to be reduced.

Moreover, when there is a free interface between two liquids there arerisks of the building of emulsions which may be stable under certainconditions, since the crude oil and the refined products may containtensoactive materials which are likely to stabilize these emul-Furthermore, when it is wished to store fuels containing water solublematerials, as this is the case for ternary fuels, the portion of thesefuels which is soluble in water will be dissolved therein, therebychanging the characteristics of this fuel.

It is possible to separate the two liquids which for example consist ofcrude oil and sea water, by means of a flexible membrane tightly securedto the periphery of the tank, thereby avoiding any contact between thetwo liquids, this membrane being of course resistant to sea water and tothe stored products, and flexible enough to be able to follow, withoutbeing subjected to any substantial stress, the variations in the levelof the interface during the filling and emptying periods of the tank. Ifhowever there is produced a stress, even only a small one, due to adifferential pressure between water and the stored product, there wouldbe a possibility, in case of an accidental piercing of the membrane,that one liquid should flow into the other and this would lead back tothe afore-mentioned problems of bacterial corrosion, of risks ofemulsions and of dissolution.

It has been proposed, prior to the present invention, to build a rigidfloating roof, similar to the metal floating roofs, which are currentlyused at the surface of the tanks, but which would have a specificgravity comprised between the respective specific gravities of the twoliquids to be separated.

Such a solution has however never been used industrially to the best ofour knowledge, since it requires in order to permit the free verticaldisplacement of said floating roof, a large peripheral clearance, thisclearance leaving a non-negligible free interface between the two fluidsto be separated, which results in the above-mentioned drawbacks:bacterial corrosion, risks of emulsions and of a dissolution of afraction contained in one phase and soluble in the other.

The main object of the invention is accordingly to provide in a tankcontaining two fluids a separation between these fluids which introducesin the separating elements no stress or at most only a negligible one,due to the differential pressure between the two fluids, avoiding anyfixation of the separating element to the periphery of the tank andleaving only an absolutely negligible interface between the two fluids.

This result is obtained, according to the invention, by inserting afloating, rigid or flexible, separating element between the storedproduct and the other fluid, said separating element being displaceablealong the vertical wall of the tank with some peripheral clearance,compensated for by means of a very flexible sealing skirt fixed to saidfloating element at its periphery, this skirt being of reduced thicknessso that it presses itself tightly against thetank wall and is capable ofturning upwardly or downwardly along the tank wall, without any mixingof the two liquids when one-proceeds from an emptying to a fillingperiod of the tank and vice versa.

The floating element may be provided with orifices for the passage offilling or emptying pipes, the edges of said orifices being alsoprovided with flexible sealing skirts.

The floating element and the skirt or skirts are so constituted as tohave each an apparent density between d and d a and d being therespective densities of the two separate liquids.

In the case where these two liquids consist respectively of crude oiland sea water, this apparent density will be selected between 0.8 and1.3 and preferably between 0.09 and 0.98.

Some non-limitative embodiments of the invention are illustrated in theattached drawings wherein FIG. 1 is a diagrammatic overall illustrationof an embodiment of the invention;

FIG. 2 illustrates on a larger scale the peripheral portion of thefloating element of FIG. 1, showing in more detail the sealing skirt;

FIG. 3 illustrates an embodiment of the central part of this theelement; decant;

FIG. 4 shows an improved embodiment of the central part of decant;floating element, in the case where the fluid stored in the tankcontains suspended products, liable to decant.,

FIG. 5 is a partial enlarged view showing how the sealing between thetwo liquids is achieved; and

FIGS. 6, 6A, 7 and 8 illustrate processes for preforming the sealingskirt.

In the drawings, I-I designates the heavier fluid (for example seawater) and L the lighter one (for example crude oil).

In the overall diagrammatic illustration of an embodiment of theinvention, shown in FIG. 1, concerning for example a floating tank 13used for offshore storage of crude oil, it is apparent that the floatingelement 1 may comprise as many sealing skirts 2 as required for thepassage of vertical pipes, for example inlet and outlet pipes for thecrude oil (pipes and 11) or pipes for discharging the rain water fromthe roof of the tank, if such a discharging device is used (pipe 12).

FIG. 2 shows the detail of the periphery of the floating element,according to FIG. 1, and particularly the flexible self-sustained skirt3 separating the two liquids, the suitable density of which is obtainedeither by choosing a homogenous material like polyethylene or apolyvinylchloride (PVC)-polypropylene compound or any other aggregate ofthermo-plastic materials and/or elastomers exhibiting the requiredproperties of flexiblity, specific gravity and resistance to theseparate fluids.

FIG. 3 shows an embodiment of the central part of the floating element,wherein a suitable average specific gravity, comprised between theabove-indicated limits is obtained by using an aggregate of materialshaving a density respectively higher and lower than the selected averagedensity, for example by constituting said floating element by two layersof glass resin stratified material 4 and 5 separated by a layer 6 ofpolyurethane foam, the specific gravity of which will be so selected asa function of the respective thicknesses of the different layers 4, 5and 6 and of the nature thereof, as to obtain the required averagedensity.

The sealing skirt or skirts may be manufactured in a simple manner byextrusion using a flat drawing plate, by extruding and inflating thematerial constituting the skirt or by any other suitable means.Thefixation of such a flexible skirt to the central part of the floatingelement may be effected by welding, sticking, or by any other suitablemechanical fixing means. This very flexible skirt of small thicknessturns upwardly or downwardly along the tank wall when one proceeds froman emptying period of the tank (position shown in solid line in FIG. 2)to a filling period of this tank (as shown in dotted line) and viceversa.

FIG. 4 shows a preferred but not limitative embodiment of the invention,wherein is used, by reason of the ease of manufacture, a homogenousmaterial having the required specific gravity.

There can be used, for example, a high density polyethylene, when crudeoil must be separated from sea water. Such a product having a density of0.95 to 0.96, lying in the above-stated density range, has an excellentchemical resistance to oil products and to sea water, resists also verywell to microbial colonies and may be employed economically usingprocesses which are conventional in the art. It is possible, inparticular, in the case of large sized floating elements to manufacturestrips by extrusion, then join these strips together either by weldingor sticking, or also, as in the illustrated embodiment, by mechanicalassembling, using an extrusion drawing plate, these strips havingprofiled parts at their ends in the form of complementary male andfemale elements for the subsequent mechanical connection of the extrudedstrips with one another.

In some cases, it might be advantageous to give the profiled strips avariable thickness, so as to constitute in a way catch-drains which makeit possible to drain the decanting products (for example sea watersuspended in the crude oil) which might settle onto the separatingelement and enabling, by means of small openings through the bottom ofthe catch-drain, to return to the lower liquid the products suspended inthe upper liquid which are heavier than this liquid.

Such an embodiment is illustrated in FIG. 4, the floating element 7being formed of strips having a regularly decreasing thickness from theedge to the center of the strip and being provided with central openings8 which are or not regularly spaced from one another, enabling thedecanting products 9 to be drained toward the lower liquid phase.

Such openings enabling the decanting products to be drained could not beprovided in a flexible wall separating the two fluids if this wall wasfixed along the periphery of the tank, because of the pressuredifference of the two fluids on both sides of this wall, which wouldlead, as already indicated, to a substantial pollution of one fluid bythe other one.

On the contrary, it is possible to provide with draining openings afloating element according to the invention, since such a floatingelement neither introduces nor is subjected to a substantial stressresulting from a difierential pressure.

Such openings will only introduces a negligible interface. For example,in a tank having a diameter equal to 10 meters, which is equiped with afloating element having no sealing skirt, according to the prior art, adistance of 4 to 5 cm must be left between the internal wall of the tankand the edge of the floating element, so as to permit free displacementof the latter.

The interface which is left free by such a floating element has an areaof about 1. 56 m for an overall cross sectional area of the tank ofabout 78.6 m and the ratio of this free interval to the overall areawill be of about 2 percent, which is far from negligible.

On the contrary, if this tank is equiped with a flexible skirt accordingto the invention, leaving at the tank periphery an interface which isabout 0.1 mm broad, the area of the interface will only be 3l.4 cm whichcorresponds to only 40 millionth of the overall area and is absolutelynegligible.

In the embodiment wherein the central part of the floating elementitself is constituted, as illustrated in FIG. 4, by l-meter broad stripsprovided in their central part with openings having a diameter of 1 cm,separated by 1 meter intervals, which may be deemed quite sufficient,the additional interface thus introduced will be 0.78 10' m for each mof the tank section, i.e. 0.0078 percent of this tank section, which isagain negligible.

FIG. 5 is an illustration on a larger scale showing how the flexibleskirt provides for sealing between the two fluids H and L. The flexibleskirt 2 according to the invention which exerts substantially no stressin the material constituting the floating element 1, when the level ofthe interface between the two liquids is stationary, also exerts only anegligible stress on this floating element when there is proceeded froma stand-by to an emptying or filling period and vice-versa, which has inparticular the advantage of avoiding any risk of a pollution of thefluids one by the other through the draining openings 8 (FIG. 4) asindicated above.

As pointed out hereinabove, this sealing skirt 2 thus turns upwardly ordownwardly along the tank wall solely under the action of thehydrostatic forces when there is proceeded from an emptying to a fillingperiod and vice versa, if this skirt is given a thickness which is smallenough.

It has been discovered that these results were obtained if the thicknessof the sealing skirt 2 was smaller In this formula, R is the radius ofcurvature, expressed in centimeters, of the tank 13 at the place wherethe skirt 2 provides for sealing, r is the maximum radius of curvatureof this skirt, expressed in centimeters, E is the average value of themodulus of elastcity for the bending of the material constituting theskirt 2, this modulus of elasticity being expressed in Newtons/m", k isa coefiicient equal to (1 d wherein d is the density of the liquid phaseH located at the lower part of the tank and d,,, is the average densityof the flexible skirt 3, (densities relative to water).

Moreover, the length 1 (FIG. 5) of the skirt 2 must be selectedsufficient so that this skirt still provides for a separation betweenthe two fluids L and H when the floating element I has been subjected toa horizontal translation and is no more centered within the tank 13.

However, for a skirt fixed along the periphery of a floating element,there will generally be selected a value of l comprised between 1r/2 rand 5 r, the value of r being smaller than the above-defined value R andbeing generally comprised between 1 and 30 cm for storage tanks of usualsizes.

The sealing skirt 2 may be manufactured with a preforming on a matrix ofsuitable shape, as hereinunder indicated.

FIG. 6 illustrates a method for manufacturing the flexible skirt 1surrounding the floating element 1.

The flexible skirt 2 is than constituted by a strip 14 previously cutout by any suitable means, so as to form regularly spaced indentations.This strip is then formed on a matrix 15. This matrix, which isdiagrammatically illustrated in FIG. 6A, has a cylindrical part 15ahaving an external diameter equal to the internal diameter 2R of thetank 13. This cylindrical part is connected to the upper part 15b of thematrix through a part 15c of toroidal shape, i.e. through a curvedportion, the radius r of which has in this embodiment a substantiallyconstant value, defining the performing radius of the sealing skirt.After the strip 14 has been placed on the matrix 15, as illustrated byFIG. 6, the adjacent cut out parts 14b are soldered together, using anyknown process, such as for example an ultrasonic soldering process, withor without addition of soldering material between these out out parts,according to the method which has been selected for cutting strip 14.

In order to achieve a preadaptation of the sealing skirt 2, whichprovide for sealing around pipes such as pipes 10, 11 or 12 (FIG. 1), totheir profile of equilibrium in operation, it is possible to alsopreform these skirts in a similar manner using a preforming matrix ofsuitable shape, such as the one illustrated in cross-section in FIG. 7,which is constituted by a cylindrical part 16a connected to a plane face16, perpendicular to the axis of this cylindrical part, through atoroidal part 160.

In the case wherein the tank has a polygonal cross section, that part ofthe sealing skirt located at the connection of two adjacent faces may bepreformed according to the above-indicated process,as illustrated byFIG. 8, by application onto a mold 17 which may be similar to matrix 15of FIG. 6 or only constituted by an angular sector of matrix 15.

Other methods for preforming the flexible skirt may obviously also beused, such as the processes of hot forming of the skirt 2 on a matrix,or also casting or molding processes, for example by injecting thematerial constituting this skirt into a mold adapted to the shape of thetank.

What we claim is l. A device for separating two liquids of differentdensities in a tank containing these two liquid one above the other,comprising a separating element of a density less than the density ofthe lower liquid and greater than the density of the upper liquidfloating between these two liquids, said separating element beingdisplaceable with some peripheral clearance along the wall of the tankand being provided at its periphery with a sealing skirt of a radiallength exceeding the distance between the separating element and thewall of the tank, wherein said sealing skirt is of small thickness andgreat flexibility and is constituted of a material having a specificgravity intermediate between those of said liquids so as to beself-sustained at the interface of said liquids and to tangentiallyengage without substantial frictional force the lateral wall of thetank, being capable of turning upwardly or downwardly along the tankwall without the mixing of the two liquids and also without anysubstantial force, when the tank is switched from a filling to anemptying operation or vice-versa.

2. A device according to claim 1, wherein said separating element isprovided with grooves for draining decanting products toward the liquidlocated below said separating element.

3. A device according to claim 1, wherein said separating element isprovided with a flexible sealing skirt which is preformed along at leasta part of its periphery, so as to be adapted to the curvature of thetank wall whereagainst this skirt is applied.

4. A device according to claim 1, wherein said separating element isprovided with at least one opening for the passage there-through of avertical pipe, the edges of these openings being also provided with asealing skirt of small thickness and great flexibility constituted of amaterial having a specific gravity intermediate between those of saidliquids so as to be selfsustaining at the interface of said liquids andto tangentially engage without substantial frictional force the lateralwall of the pipe, being capable of turning upwardly or downwardly alongthe pipe wall without the mixing of the two liquids and also without anysubstantial force, when the tank is switched from a filling to anemptying operation or vice-versa.

5. A device according to claim 4, wherein the sealing skirts with whichare provided said orifices for the passage of the pipes are preadaptedto their profile of equalibrium in operation.

6. A device for separating two liquids of different densities in a tankcontaining these two liquids one above the other, at least one of whichconstitutes a stored product, including a separating element of adensity less than the density of the lower liquid and greater than thedensity of the upper liquid floating between these liquids, saidseparating element being displaceable with some peripheral clearancealong the vertical wall of the tank, wherein said separating element isprovided at its periphery with a very flexible sealing self-sustainedskirt of reduced thickness and a specific gravity between those of saidliquids capable of applying itself tightly against the vertical wall ofthe tank and of turning upwardly or downwardly along the tank wallwithout mixing of the two liquids, when there is proceeded from afilling to an emptying period of the tank and vice versa, said sealingskirt having a thickness at most equal to the value given by theformula:

wherein E represents the average value of the modulus of elasticity forthe bending of the material constituting said sealing skirt expressed inNewtons/m'-" R is the radius of curvature of the tank wall and r themaximum radius of the skirt at the place where this skirt provides forsealing, the radii R and r being both expressed in centimeters and k isa coefiicient equal to d d,,,, 11 being the density of the heaviest ofthe two liquids separated by said flexible sealing skirt and d,,, beingthe average density of this skirt.

1. A device for separating two liquids of different densities in a tankcontaining these two liquid one above the other, comprising a separatingelement of a density less than the density of the lower liquid andgreater than the density of the upper liquid floating between these twoliquids, said separating element being displaceable with some peripheralclearance along the wall of the tank and being prOvided at its peripherywith a sealing skirt of a radial length exceeding the distance betweenthe separating element and the wall of the tank, wherein said sealingskirt is of small thickness and great flexibility and is constituted ofa material having a specific gravity intermediate between those of saidliquids so as to be self-sustained at the interface of said liquids andto tangentially engage without substantial frictional force the lateralwall of the tank, being capable of turning upwardly or downwardly alongthe tank wall without the mixing of the two liquids and also without anysubstantial force, when the tank is switched from a filling to anemptying operation or vice-versa.
 2. A device according to claim 1,wherein said separating element is provided with grooves for drainingdecanting products toward the liquid located below said separatingelement.
 3. A device according to claim 1, wherein said separatingelement is provided with a flexible sealing skirt which is preformedalong at least a part of its periphery, so as to be adapted to thecurvature of the tank wall whereagainst this skirt is applied.
 4. Adevice according to claim 1, wherein said separating element is providedwith at least one opening for the passage there-through of a verticalpipe, the edges of these openings being also provided with a sealingskirt of small thickness and great flexibility constituted of a materialhaving a specific gravity intermediate between those of said liquids soas to be self-sustaining at the interface of said liquids and totangentially engage without substantial frictional force the lateralwall of the pipe, being capable of turning upwardly or downwardly alongthe pipe wall without the mixing of the two liquids and also without anysubstantial force, when the tank is switched from a filling to anemptying operation or vice-versa.
 5. A device according to claim 4,wherein the sealing skirts with which are provided said orifices for thepassage of the pipes are preadapted to their profile of equalibrium inoperation.
 6. A device for separating two liquids of different densitiesin a tank containing these two liquids one above the other, at least oneof which constitutes a stored product, including a separating element ofa density less than the density of the lower liquid and greater than thedensity of the upper liquid floating between these liquids, saidseparating element being displaceable with some peripheral clearancealong the vertical wall of the tank, wherein said separating element isprovided at its periphery with a very flexible sealing self-sustainedskirt of reduced thickness and a specific gravity between those of saidliquids capable of applying itself tightly against the vertical wall ofthe tank and of turning upwardly or downwardly along the tank wallwithout mixing of the two liquids, when there is proceeded from afilling to an emptying period of the tank and vice versa, said sealingskirt having a thickness at most equal to the value given by theformula: wherein E represents the average value of the modulus ofelasticity for the bending of the material constituting said sealingskirt expressed in Newtons/m2, R is the radius of curvature of the tankwall and r the maximum radius of the skirt at the place where this skirtprovides for sealing, the radii R and r being both expressed incentimeters and k is a coefficient equal to d2 - dm, d2 being thedensity of the heaviest of the two liquids separated by said flexiblesealing skirt and dm being the average density of this skirt.